1. Field of the Invention
The present invention relates to a three-dimensional shape data processing apparatus and a three-dimensional shape data processing method configured to process three-dimensional shape data.
2. Description of the Related Art
During a medical examination, a doctor observes the state and the change with time of a lesion of a subject by reading an X-ray medical image displayed on a monitor, which is captured by imaging the lesion of a subject (object). In a medical examination, a tomographic image of the inside of a subject is often used as the medical image. A tomographic image is captured by using a modality.
As a modality, an apparatus, such as an optical coherence tomography (OCT), an ultrasonic image diagnostic apparatus, a magnetic resonance imaging apparatus (MRI), or an X-ray computed tomographic imaging apparatus (an X-ray computed tomography (CT)), is used.
However, it is difficult for a doctor to recognize the three-dimensional shape and the dimension or magnitude of a lesion only by observing each tomographic image captured by the above-described modality. In order to address the problem like this, a conventional method restores three-dimensional shape data based on a plurality of tomographic images. By observing a three-dimensional tomographic image generated based on restored three-dimensional shape data, it becomes easy for a doctor to recognize the three-dimensional shape and the dimension or the magnitude of a lesion by executing analysis by looking closely at the displayed image.
The above-described modality, such as an OCT, MRI, or X-ray CT captures a group of tomographic images (i.e., a plurality of tomographic images) of a lesion at regular positional intervals. Accordingly, by simply displaying the tomographic images in a mutually superposed manner, three-dimensional shape data can be easily restored.
On the other hand, if an ultrasonic image diagnostic apparatus is used as the modality, a doctor or a medical technician usually captures a tomographic image of a lesion by freely moving a probe by hand. Therefore, in this case, information about the position of the captured tomographic image within the body of the subject is not acquired. In order to address the above-described problem, a conventional method measures the position and orientation of the probe by using an external sensor and calculates the positional relationship among captured tomographic images to restore three-dimensional shape data.
However, in this case, displacement may occur among tomographic images due to movement of the subject that may occur during capturing of the tomographic images and also due to an error in estimation of spatial positional relationship among the captured tomographic images. As a result, distortion may occur on restored three-dimensional shape data.
FIG. 9 illustrates an example of distortion that may occur on restored three-dimensional shape data in a conventional method. Referring to FIG. 9, distortion (deformation) has occurred on three-dimensional shape data of an object having a shape of a cuboid (or a cube).
A conventional method reduces distortion of three-dimensional shape data by correcting displacement among tomographic images. More specifically, a conventional method corrects displacement among tomographic images, which are acquired by radially capturing images of the fundus of the eye of a subject by using an OCT. The conventional method uses one reference tomographic image of a range of a lesion in a cylinder-like shape. In addition, the conventional method minimizes the difference between the heights of contours of a captured tomographic image and a reference tomographic image by using a genetic algorithm. Thus, the conventional method acquires highly accurate data of the shape of the fundus.
Another conventional method corrects displacement among tomographic images captured by an ultrasonic image diagnostic apparatus by using one reference tomographic image, which is captured in a direction perpendicular to the direction of capturing the tomographic images. This conventional method corrects displacement among ultrasonic tomographic images by aligning a captured tomographic image and the reference tomographic image based on a result of comparison of pixel values of the captured tomographic image and the reference tomographic image on an intersection thereof.
Yet another conventional method improves the quality of a tomographic image by combining a plurality of pieces of three-dimensional shape data. More specifically, Japanese Patent Application Laid-Open No. 06-259533 discusses a method for combining and acquiring highly accurate three-dimensional shape data based on a plurality of sectional images of a subject, which is captured in a plurality of directions while at different in-focus positions of a microscope. This conventional method addresses such a problem that the characteristic of spatial frequency in the direction of an optical axis is lower than that in the direction within the image plane by combining sectional images acquired in different optical axis directions.
In addition, Japanese Patent Application Laid-Open No. 2000-207574 discusses the following method. The conventional method acquires a plurality of pieces of volume data by scanning a subject in a plurality of different directions and generates surface data based on the plurality of pieces of volume data. Furthermore, the conventional method deforms and combines the plurality of pieces of surface data. The conventional method also addresses such a problem that the characteristic of spatial frequency in the direction of scanning is lower than that in the direction within the image plane by complementarily using data obtained in different scanning directions.
However, if distortion has occurred on a reference image itself due to movement of a subject during capturing of the reference image, any of the above-described conventional methods cannot correct the distortion on the reference image. Furthermore, each of the above-described conventional methods is directed only to correct displacement among tomographic images. Accordingly, each of the above-described conventional methods does not satisfy a desire by the market for improving the image quality of three-dimensional shape data.
In addition, in each of the above-described conventional method, it is assumed that a subject remains stationary. Accordingly, if a subject moves during an image capturing operation, each of the above-described conventional methods cannot acquire three-dimensional shape data of a sufficient image quality. Furthermore, if distortion has occurred on three-dimensional shape data due to movement of a subject during processing for acquiring each three-dimensional shape data, each of the above-described conventional methods cannot correct the distortion on the three-dimensional shape data.